If you find difficulties enabling or installing Gnome Shell themes under Ubuntu 12.04 LTS Precise Pangolin, then this tutorial will be useful for you. Before we start, you should know that this tutorial is about Gnome Shell themes not GTK+ themes because they are totally different.

Then all downloaded themes must be extracted there. We will use as an example the installation of the "Flat And Square" GNOME Shell theme, you can download it from here. Extract the theme contents to the ".themes" folder:

Start now Gnome Tweak Tool and head to the Theme tab and select the "Flat And Square" theme from the dropdown menu to enable it:

Note: You can also use the browse button provided in Gnome Tweak Tool to install and enable Gnome Shell themes.